In prior art telecommunications switching systems comprising a plurality of stored program controlled switch nodes, it is well known that when two switch nodes are interconnected by a plurality of communication links, to form these communication links into a common group that share a signaling channel. For example, ISDN PRI links are formed into a non-facility access service (NFAS) group. Within a NFAS group of PRI links, the D channels (which are normally used for signaling) in all but two of the PRI links are utilized as an additional B (bearer) channel. In the two remaining PRI links, one D channel is designated as the primary D channel, and the D channel of the second remaining PRI link is designated as the secondary D channel. In accordance with CCITT specifications, if the primary D channel is lost, then, the two switching nodes will utilize the secondary D channel for signaling. In accordance with the CCITT specification, if a PRI link is lost no attempt is made to preserve the calls on B channels of the failed PRI link by moving them to B channels of other PRI links within the NFAS group. This results in calls being lost even though there is spare capacity within the NFAS group to continue these calls.
There are a number of reasons why the primary PRI link may be disabled. One reason is that the primary PRI link is a wired link that has been cut accidentally. A second reason is that in a highly distributed system such as illustrated in U.S. Pat. No. 5,390,242 it is necessary to add and remove PRI links interconnecting switch nodes. At present, if calls are not to be terminated, it is necessary to wait until there are no calls being communicated on any of the links making up the NFAS group. This is often done by indicating that each of the links is out of service. In general, this requires field personnel to work during the early morning hours when there is little activity on the system.
In addition to wired PRI links, other links could be utilized such as wireless links. In the case of the wireless links, interruptions of the wireless path can occur which can cause the loss of one or more links of the NFAS group.
It is clear that a problem exists in the art with the present procedure of simply dropping calls on B channels of a failed link. As noted in the previous paragraph, there are a variety of conditions under which such failures can happen on a routine basis, and it is not desirable to lose routine calls.